Z32: Don’t install blow off valves on a 300ZX

Tech Article: Don’t Install a BOV on a 300ZX
Composed by: 300ZXttZman
By: Greg @ Specialty Z

Here’s why you won’t want to install BOV’s on a engine with a MAF sensor: When you make any positive boost and you close the throttle without a BOV or recirculation plumbing, the pressurized air will stop moving and stall the turbo.

Blow off valve:
What a BOV does is open a valve when you let off the throttle by way of engine vacuum, this allows the air to be released into the atmosphere. This is better for the turbo than nothing in the system because it allows to turbo to keep spinning.

The problem with a blow off valve in a MAF sensor set up is that because the turbo is still spinning, new air is being pulled into the turbo from the air filter and is being measured by the MAF sensor. So the ECU believes all the air coming into MAF is going into the engine that is now being pushed out into open air. So when you hear that sound of air rushing, the engine is pumping fuel in for all that air you hear coming out. This causes the engine to want to stall, because it is being flooded with fuel.

The engine wanting to die is one thing, the other is that all the unburned fuel going into the cylinders will not burn and this washes the lubrication off the cylinder walls and the engine rings, creating premature engine wear.

The plumbing on a Z with MAF’s is not designed to work with a BOV and you can not tune it out. A MAF measures the air being drawn past the air filter to allow the ECU to calculate the fuel needed. The MAF needs to be measuring the air and the ECU has no way of knowing when you’re blowing some of the air outside the engine.

What a MAP sensor does is measure vacuum and pressure in the intake manifold to tell the engine management how much fuel is needed. So it does not matter what happens in the pipes before the throttle body with a MAP system, as the MAP only measures what happens in the engine and not what is happening BEFORE the manifold. BOV’s work fine on MAP set ups, but a recirc valve is always better, as it is helping to keep the turbo spinning by feeding the turbo air.

Recirculation Valve:
The stock Nissan recirc valve is larger than most aftermarket BOV’s. The stock Nissan BOV is also stronger than most aftermarket BOV’s. The stock recirc’s can handle more than 40 PSI boost pressure. What a recirc valve does is similar to a BOV, but better. Just like a BOV the recirc valve opens when it gets a vacuum signal – the difference is that instead of allowing the air to be blown off into the atmosphere, the recirc valve blows the air back into the front side of the turbo.

What this does is let the air out from the pressure side of the turbo and blows that air back into the suction side of the turbo. The air pushing back into the front side of the turbo is helping it keep spinning because it is recirculating the air. The air simply goes out of the turbo and right back in.
Because the air that is coming out of the turbo is going right back into the turbo, no new air is being pulled in through the MAF because the air is going in a loop.

Removing the stock recirc makes zero sense and will only cause issues with stalling and filling your crankcase with fuel. As long as you run a MAF you should never install a BOV.

Now, the reason some BOV’s cause less problems is that they come with different springs, a heavier spring requires more vacuum to open the valve. The problem with too stiff of a spring, is the valve is not opening enough, this will cause compressor surge and is hard on the turbos.

Special thanks to Greg @ Specialty Z

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